This invention relates to a novel process for the polymerization of 1,3-butadiene employing an organolithium-Lewis base catalyst system and to novel polybutadienes thus produced.
It is conventional to polymerize butadiene in an inert diluent employing an organolithium compound as catalyst. In the known process, polybutadienes are obtained having a 10% vinyl group content.
It is also known that polybutadienes having higher vinyl group contents can be produced by the addition to the catalyst system of a suitable Lewis base, such as, for example, ether (I. Kuntz, A. Gerber, J. Polym. Sci. 42, 299 [1960]), Depending on the type and amount of the Lewis base added, polybutadienes of any desired vinyl group content between 11 and 88% can be produced. (Dutch Published Patent Application No. 68,09,874). The vinyl group content of the polybutadienes does not depend solely on the type and amount of the Lewis base employed. If a higher polymerization temperature is employed, a polymer with a lower vinyl group content is obtained. (A.W. Langer; A. Chem, Soc. Div. Polymer Chem. Reprints; Vol. 7 (1), 132 [1966]).
In the previously known processes for the polymerization of butadiene in the presence of a Lewis base, the polymerization is conducted isothermically, i.e., at a constant polymerization temperature. Under these conditions, polybutadienes are obtained having vinyl groups which are uniformly distributed along the longitudinal axis of the macromolecule. This uniform distribution of the vinyl groups is based on long-range order (order-disorder phenomenon).
The polybutadiene homopolymers produced by an isothermic polymerization employing any particular catalyst system varies according to the selected polymerization temperature. However, whatever the selected temperature, the resultant homopolymer does not have optimum properties for certain end-uses.
As disclosed in the copending application of Karl-Heinz Nordsiek et al. entitled "Tread Strips for Tires Formed of Homopolymers of Polybutadiene," S.N. 92,537, filed November 24, 1970, now Pat. No. 3,741,927, issued June 26, 1973, butadiene-styrene copolymers are conventionally employed in the rubber blends used to produce tire tread in order to obtain a vulcanizate having both acceptable wearing properties and skid resistance and because conventional polybutadiene homopolymers have poor processing properties. Thus, although processing economies would result if a polybutadiene homopolymer could be employed as the prepolymer in the production of tire tread vulcanizates, heretofore rubber blends have been used.
It has now been found that novel polybutadiene homopolymers having the requisite processing conditions and whose vulcanizates have the desired optimum combination of properties for many end-uses, especially tire treads, without blending with other rubbers can be produced according to the process of this invention.